Summary:
In E.coli K-12, PhoRB is the two-component system for sensing and responding to variations in the level of extracellular inorganic phosphate (Pi). Pho RB regulates the expression of a large number of genes (the Pho regulon) which have a role in the assimilation of environmental Pi or the use of alternate Pi sources.

PhoR is the membrane associated sensor kinase which controls activity of the cytoplasmic response regulator and transcription factor PhoB. PhoR detects Pi only indirectly and its activity as a sensor kinase depends on its association with the phosphate transporter PstSCAB and/or PhoU. Accordingly signal transduction by Pi requires seven proteins (PhoR, PhoB, PstSCAB and PhoU) that interact in a membrane associated complex.

PhoRB signalling is believed to be a negative process - that is, activation of PhoB is the default state and excess environmental Pi is required to turn the system off. Under limiting environmental Pi, PhoR is believed to be in an active signalling conformation resulting in the phosphorylation and activation of PhoB and subsequent induction of the Pho regulon. Excess levels of Pi (>4uM) result in an inhibited form of PhoR which interferes with phosphorylation of PhoB and prevents its activity as a transcription factor. The Pst transporter and the chaperone like protein PhoU are both necessary for inhibition of the PhoRB system.